Stage 2: once pyruvate has been produced, two alternative pathways can occur. In plants and many microorganisms the pyruvate is broken down to ethanol via ethanal (acetaldehyde) in a process called ALCOHOLIC FERMENTATION, which requires hydrogen from NADH (see Fig. 34 ). In animals the pyruvate is changed via a single step into LACTIC ACID, a process called lactic-acid fermentation, which again requires hydrogen from NADH.

The role of NADH in both fermentations should be noted. Since the amount of NAD present in the cell is limited, glycolysis would quickly come to a halt if anaerobic respiration stopped at pyruvate. By going on to ethanol or lactic acid, NAD is freed in the fermentations to return to glycolysis and thus allow glucose CATABOLISM to continue. The yield of ATP in anaerobic respiration is poor because, firstly, the ETS cannot be used without oxygen, and, secondly, the end products still contain large amounts of energy. As a result, the free energy released and stored is only a fraction of the amount in the complete oxidation of glucose, as is shown in Fig. 35.

anaerobic respiration,

n a cellular process that occurs in the absence of free oxygen to partially break down glucose molecules and provide a small net gain of ATP.

respiration

1. the exchange of oxygen and carbon dioxide between the atmosphere and the body cells, including inspiration and expiration, diffusion of oxygen from the pulmonary alveoli to the blood and of carbon dioxide from the blood to the alveoli, and the transport of oxygen to and carbon dioxide from the body cells.

2. cellular respiration, the metabolic processes by which living cells break down carbohydrates, amino acids and fats to produce energy in the form of ATP (adenosine triphosphate).

abdominal respiration

inspiration and expiration accomplished mainly by the abdominal muscles and diaphragm. Occurs in acute pleurisy because of pain in the chest and fixation of the thorax, and tick paralysis due to paralysis of the intercostal muscles.

aerobic respiration

oxidative transformation of certain substrates into secretory products, the released energy being used in the process of assimilation.

anaerobic respiration

respiration in which energy is released by chemical reactions in which free oxygen takes no part.

artificial respiration

that maintained by force applied to the body. Called also artificially assisted respiration.

the processes in the living cell by which organic substances are oxidized and chemical energy is released.

Cheyne-Stokes respiration

breathing characterized by rhythmic waxing and waning of respiration depth, with regularly recurring apneic periods. See also cheyne-stokes respiration.

cogwheel respiration

breathing with jerky inspiration.

controlled respiration

during general anesthesia using an endotracheal tube with an inflated cuff, the animal's respiration can be controlled completely by compression alternating with relaxation on the rebreathing bag of the breathing circuit. See also intermittent positive-pressure ventilation.

costal respiration

the respiratory movements are mostly carried out by the chest wall.

diaphragmatic respiration

that performed mainly by the diaphragm.

electrophrenic respiration

induction of respiration by electric stimulation of the phrenic nerve.

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